1. Field of the Invention
The present invention relates to a method and apparatus for the growth of a single crystal, especially, .beta.-type barium borate (.beta.-BaB.sub.2 O.sub.4).
2. Description of the Related Art
There has been a demand for a laser light source with shorter wavelengths in recent years to satisfy the requirements of higher recording density and precise processing in optically technical fields, e.g. optical record playback, precision processing or the like.
As one of the measures satisfying the above demand, animated research and development are now being made to obtain a laser light with a short wavelength by frequency conversion of the laser light from a conventional solid laser utilizing a nonlinear optical material.
A large nonlinear optical effect and high transparency, namely, excellent optical properties with small light loss and high resistance to laser damages are demanded of the nonlinear optical materials.
Among these nonlinear optical materials, a .beta.-BaB.sub.2 O.sub.4 single crystal is characterized in that it can convert into wavelengths ranging up to a UV (ultraviolet) region and is resistant to laser damages. Hence for the .beta.-BaB.sub.2 O.sub.4 single crystal, the growth of larger crystals of high quality is desired.
In view of this situation, energetic studies of the .beta.-BaB.sub.2 O.sub.4 monocrystal growth are ongoing in various fields. The barium borate (.beta.-BaB.sub.2 O.sub.4) crystal, though it is a congruent melting composition, has the feature in the presence of two phases, a high temperature phase consisting of .alpha.-type barium borate(.alpha.-BaB.sub.2 O.sub.4) and a low temperature phase consisting of .beta.-type barium borate (.beta.-BaB.sub.2 O.sub.4) and the transformation temperature between two phases is about 925.degree. C. (JIANG Aidong et al, Journal of Crystal Growth 79 (1986) 963-969 etc.). Hence the growth of the low temperature phase .beta.-BaB.sub.2 O.sub.4 crystal is regarded to be difficult in the ordinary lifting growth method since the high temperature phase .alpha.-BaB.sub.2 O.sub.4 crystal is crystallized in this growth method.
The crystal growth is therefore attained in customarily usual methods such as a TSSG (Top Seeded Solution Growth) method which is a type of flux method and a pulling method using a flux under the condition in which the low temperature phase .beta.-BaB.sub.2 O.sub.4 crystal is crystallized as the primary crystal by using a flux such as Na.sub.2 O or the like to lower the melting point or a pulling method using a flux or the like. However, these methods pose the problems of low productivity due to the extremely low crystal growth rate and of deteriorated optical properties due to contamination with flux components in the crystal.
While, Japanese Patent Application Laid-Open (JP-A) No. H1-249698 proposes a pulling method using a self-flux in which external or different kind of fluxes are excluded and excess barium or boron is added to a barium borate composition. This method is improved in terms of exclusion of impurities of a different nature. It, however, has the problem that since a melt composition largely differs from the grown crystal composition, the melt composition varies with the progress of the growth and hence it is difficult to produce large and homogenous crystals.
On the other hand, there has been a recent report in which a .beta.-BaB.sub.2 O.sub.4 single crystal can be grown by a lifting method even from a melt of barium borate without using a flux at all (JP-A No. H2-279583). In this method, using a high frequency induction heater, induction current is produced in a crucible itself and the crucible is directly heated to set the ambient temperature condition just above the melt, specifically, to form a large temperature gradient just above the melt intentionally thereby growing a .beta.-BaB.sub.2 O.sub.4 crystal.
Another method is proposed (JP-A No. H9-235198) in which a high frequency induction heater is used similarly and no flux is used. In this method, although the crystal is pulled from a melt of barium borate, the growth of .beta.-BaB.sub.2 O.sub.4 crystals by pulling can be attained even if the temperature gradient just above the melt is not so steep, specifically, a difference between the temperatures at a height of 10 mm above the melt surface and at the melt surface is between -165.degree. C. and -280.degree. C., in other words, even if the temperature gradient upward from the surface of the melt is as relatively gentle as 165.degree. C./cm to 280.degree. C./cm. Besides, by rather selecting relatively gentle temperature gradient in this manner, this method is improved in the prevention of generations of cracks and striae caused by the strain of the grown crystal which tends to be produced when the temperature gradient above the melt is made steep.
However, any of the aforementioned crystal growth methods using no flux does not necessarily satisfy the internal optical uniformity of the grown .beta.-BaB.sub.2 O.sub.4 crystal, particularly when the crystal growth method intends to produce large diameter crystals.